1. Field of the Invention
The present invention relates, generally, to a mass flow controller, and more particularly to a controller for controlling a mass flow of a fluid by measuring the mass flow of the fluid and then by comparing the measured mass flow to a standard flow.
2. Description of the Related Art
Generally, various kinds of gases are used in semiconductor fabricating processes. A mass flow controller controls mass flows of the gases. As semiconductor devices have become more highly integrated, requirements for controls to accurately measure and control the mass flows of gases to semiconductor fabricating processes have increased.
FIG. 1 is a sectional view illustrating a conventional mass flow controller, and FIG. 2 is a partially enlarged view illustrating a mass flow sensor of the conventional mass flow controller of FIG. 1.
Referring to FIGS. 1 and 2, a base 110 of a conventional mass flow controller 100 has a passage 112 through which a fluid passes, an inlet portion 114 for introducing the fluid into the passage 112 and an outlet portion 116 for releasing the fluid from the passage 112.
A bypass portion 120 through which the fluid passes is formed in the passage 112 adjacent to the inlet portion 114. A sampling pipe 132 is connected to the passage 112. Particularly, the sampling pipe 132 is connected between a first portion adjacent to an inlet end of the bypass portion 120 and a second portion adjacent to an outlet portion of the bypass portion 120, thereby allowing a sample of the fluid passing through the bypass portion 120 to pass through the sampling pipe 132.
A mass flow sensor 130 measures a mass flow of the fluid passing through the bypass portion 120. The mass flow sensor 130 includes a first thermal resistance 134a and a second thermal resistance 134b, wherein the first thermal resistance 134a and the second thermal resistance 134b are wound around the sampling pipe 132. The first thermal resistance 134a and the second thermal resistance 134b comprising Pt or other metals similar to Pt are connected to a bridge circuit 136. A control valve 140, e.g., a solenoid valve, is connected between the bypass portion 120 and the outlet portion 116.
When the first thermal resistance 134a and the second thermal resistance 134b are heated, a temperature difference proportional to the mass flow of the fluid is generated between an upper stream and a lower stream of the sampling pipe 132. Therefore, resistance values of the first thermal resistance 134a and the second thermal resistance 134b are different from each other. The bridge circuit 136 detects the different resistance values as an electric signal. The detected signal is amplified through an amplifier (not shown). The compensator compensates the amplified signal to correspond to the mass flow of the fluid.
The measured signal indicating the mass flow of the fluid is transmitted to a valve controller (not shown). The valve controller compares the measured signal to a predetermined standard flow. The valve controller controls operations of a control valve 140 to correspond the measured signal to the standard signal.
Full scale of the mass flow of the fluid is determined according to a volume of the bypass portion 120. The full scale may not be readily controlled. In the meantime, the semiconductor fabricating processes contain various kinds of lot processes. Also, various kinds of gases are used in the fabricating processes. In addition, the gases supplied to a semiconductor substrate to perform the fabricating processes have different mass flows from one other. Accordingly, there exists a problem that the conventional mass flow controller 100 may not be applied to the various processes.
Therefore, a need exist for a mass flow control that can control a full scale of a mass flow and the mass flow of the various kinds of gases used in the various processes of semiconductor fabrication.